This invention relates to an article of footwear.
The desirability of providing the ground-engaging part, more especially the heel, of an article of footwear, with a shock-absorption capacity greater than is practicably obtainable by the resilience of the material used in the construction of the article per se has been recognised for many years. To make walking on hard pavements, for example, more comfortable it has been proposed to incorporate helical metal compression springs or gas springs. For example, U.K. patent specification No. 200,368 of 1922 proposes the use of air springs in both the sole and heel regions of a boot. The problem remains, however, that comfort is determined by the relationship between the weight of the wearer, the hardness of the surface on which he is walking and the rate of the spring medium. It is a sine qua non that the spring medium should be capable of restoring the shape of the unloaded article between steps, but this can be achieved with a spring medium of such low shock-absorption as to represent very little improvement in wearer comfort. On the other hand a high shock-absorption characteristic is achieved with a relatively "hard" spring which may cause discomfort through excessive "bounce".
Prior art efforts to overcome this problem, i.e. to adapt the spring rate to the particular user, have involved (a) in the case of gas springs the introduction or exhaustion of gas (for example U.K. patent specification No. 200,368) and (b) in the case of rubber or metal springs their replacement with springs of different rate (for example U.K. patent specifications Nos. 390,368 and 427,126). Neither of these solutions is satisfactory because a source of compressed gas is not always readily available, and whereas metal springs suited to the user's weight may be selected they cannot readily be changed whenever he walks on different surfaces. These operations are in any case elaborate and expensive and not easily carried out by the wearer as occasion demands and without access to special equipment or accessories.
U.K. patent specification No. 456,979 discloses a heel sprung by a helical compression spring the rate of which can be varied by turning a screw. Here, however, the same screw determines the limit of telescopic extension of the two parts of the heel and consequently any variation of the spring rate will also affect the height of the unloaded heel and alter the extent of telescopic extension of the two heel components. U.K. patent specification No. 456,979 is not regarded by the present applicant as a practicable proposal, for example because of the manifest danger of loosening of the woodscrew, but even with improvements of a non-inventive character this specification does not disclose a sprung heel which can be readily adjusted by the wearer without affecting characteristics of the heel distinct from the rate of the spring.